Increasing traffic density and greater development of open spaces restrict traffic space more and more, especially in congested urban areas. The available parking space becomes tighter, so that the driver has to cope not only with the demands of increasing traffic volume, but faces additional stress from the search for a suitable parking space. Estimating the exact size and position of the parking space, in particular during back-up parking into a parking space, is often quite difficult.
All types of devices are known to make parking easier for the driver of a vehicle, for instance by parking space measurement, or a semi-autonomous or fully autonomous parking assistant.
Once a suitable parking space has been located, the semi-autonomous parking system calculates the optimal maneuvering path as a function of the position and size of the parking space. With the aid of a suitable interface (optical, acoustical or haptical), the driver is given information about the way he has to steer and accelerate or brake in order to enter the parking space in an optimal manner. The deviations from the setpoint trajectory are compensated by the driver himself or adjusted automatically. The longitudinal movement is generally determined by the driver. Steering may also be implemented automatically, the driver being responsible merely for accelerating and braking.
In order to be able to calculate the setpoint trajectory of the maneuvering path, both the position of the parking-space delimiters must be determined very accurately and the depth and shape of the parking space has to be determined very precisely.
A device for determining a parking-space boundary is known from U.S. Pat. No. 5,701,122. Optical sensors are utilized to sense the distance of a fixed object. However, the aforementioned approach has the disadvantage that the use of optical sensors causes considerable expense. Furthermore, it is disadvantageous that the optical sensors are highly susceptible to failure, especially when the vehicle exterior is dirty.
To overcome the aforementioned disadvantages, it is known to utilize ultrasonic sensors for the detection or for the distance measurement of objects in the vicinity of a vehicle, since these sensors are relatively inexpensive. A disadvantage of the known systems for parking space measurement space based on ultrasound is their low accuracy, which considerably restricts their practical use. Especially when passing the parking space to be measured at a high speed (up to 30 km/h), the accuracy of the known parking-space measurement systems based on ultrasound is not up to the task.